This invention relates to silicate glass compositions containing CaO which provide a substitute for the high lead content glasses now commonly used in the spacing apart of the electrical in-lead components of electric lamps and similar devices. In the manufacture of electric lamps such as incandescent and fluorescent type lamps as well as television picture tubes and other electron discharge devices it is conventional to employ glass tubing in the form of stems, exhaust tubes, and flares to support and hermetically seal the electrical in-leads within a sealed glass envelope. The prior art glasses containing as much as 30% by weight PbO which are being used in this manner exhibit a volume electrical resistivity high enough to prevent conductivity between the spaced apart in-leads during operation of the particular electrical device. The prior art glasses also exhibit a long enough softening range to permit hand or relatively slow machine speeds to be employed in device manufacture.
On the other hand, the lead oxide constituent of these prior art glasses has always been relatively high priced. It is toxicologically hazardous and when handling a glass batch containing 20-30% PbO produces dust and fumes which exceed statutory emission limits and represent the loss of high-priced raw material. During melting of such glass batches, there is also a tendency of inhomogeneous striae of glass containing higher amounts of lead oxide to settle to the bottom of the glass tank by reason of higher density compared with other homogeneous glass in the furnace and this leads to poor homogeneity glass in the final glass and size fluctuations in the tubing as drawn. Poor mixing and segregation in the glass tank necessitates longer melting and refining times than would be needed for glasses having a lower lead oxide content. Another problem encountered with the high lead content glasses during melting is attributable to high viscosity in the melting temperature region (1425.degree. to 1525.degree. C). Reduction of the glass viscosity would permit shorter melting and homogenizing times to be used. Still a different problem is encountered with high lead glass tubing during device manufacture whereby the very long working range necessitates slower device forming machine speeds than would otherwise be needed with a shorter working range glass. Understandably, higher cost is also associated in utilizing high lead content glasses since the higher density of these glasses produces less tubing of a given size than is possible with the same weight of glass having a lower density.